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<br />- TIIE CA\I JIA RAY AND SPECTRAL CAMHA R:\Y LOCS - <br />In practical terms, the compromise is that the simple <br />gamma ray tool should not navel more than 30 cm in the <br />time constant, Table 7.5 sets out the limitations of [ime- <br />constants and logging speeds. However, even following <br />these constraints, gamma ray log repeatability in the <br />minor variations is poor. This is caused by a combination <br />of the logging method, the computing method and the <br />natural statistical variation of gamma ray emissions. A <br />comparison between a main log and a repeat section or <br />between separate runs of the gamma ray tool, gives an <br />empirical indication of the extent of the problem. [t only <br />involves the small scale variations. <br />Table 7.6 Specval tool logging, time constants. <br />Time constant Logging Formation Company <br />(seconds} speed logged in <br />time constant <br />(m/h) (cm) <br />4 275 30.5 Schlumberger <br />6-8 183 30.5-~0.6 Western Atlas <br />depth related 275 n/a B.P.B. <br />SPECTRAL GAMMA RAY <br />The specval tool is sensitive to speed effects, essen- <br />tially because the count rate of the individual detection <br />windows is very small. Because the time constant is <br />generally fixed, the significant variable is the logging <br />speed. Lower speeds allow higher counts per formation <br />interval. Tools in which the low energy specwm is used <br />are as a rule run faster than tools in which the loin energy <br />sptctrum is not used. Table 7.6 gives recommended <br />speeds. Despite these recommendations, many, if not <br />most spectral logs have poor repeatability (Hurst, 1990 <br />and Figure 7.9), which may be a result of lossine speed <br />rather than any inherent error. It is good practice to run a <br />repeat section with every spectral log so that the variability <br />can be judged. Moreover, interpretation methods must <br />take the quantitative level of variability into account. <br />Unwanted borehole effects <br />Simple gamma ray -The simple eatnma ray log is rela- <br />tively unaffected by small-scale borehole irregularities, <br />but is affected by large caves (Table 7.7). The effect is <br />due to the increased volume of drilling mud between the <br />formation and the gamma ray detector which causes <br />increased Compton scattering and a consequent diminu- <br />tion in the gamma ray log value, as described previously. <br />Most logging companies publish charts to correct for <br />borehole size with mud weight consideration. <br />A quite different effect is caused by the use of the <br />radioactive mud additive KCI. The potassium radio- <br />activity of the KCI is detected by the gamma ray tool and <br />the usual result is a marked increase in the absolute <br />GAMMA RAY API <br />tso <br />E <br />c <br />m <br />t <br />_ <br />4= ~ <br />,--- <br />~ --- <br />- <br /> <br />-, ---=_= <br /> °- ~ <br /> c <br />- <br /> ` <br /> ;____ <br />Well 1 <br />x ` <br />° Well <br />o r- <br />~ ~ ~ <br />~ KCI mud <br />A r <br />i <br />--- repeat <br />Figure 7.9 Repeatability of the spectral gamma ray. Precise <br />repeatabiliq• is generally poor but it should be noted that the <br />quantities bring detected are very small. <br />Figure 7.10 The e[fect of KCI in the drilling mud on gamma <br />ray values. Well I, with ordinary mud, well "! with KCI mud, <br />The formation values should be tht same. ~, is the difference <br />created by tht KCI content. Tht wells are 3km span. <br />73 <br />- main run <br />